Field of Endeavor
The present invention relates to the field of combustion technology, especially for gas turbines. It refers to a method for operating a combustion device and also to a combustion device for carrying out the method.
Brief Description of the Related Art
In combustion chambers with a plurality of burners operating in parallel, as occur in gas turbines, piston engines, and boilers, the flame temperatures of the individual burners are balanced or homogenized for maximizing service life and for minimizing pollutant emission. This homogenization is customarily constructionally achieved by a construction of the individual combustion chambers and their fuel supply which is as identical as possible. However, in the realized system, temperature differences between the burners, which lie above the tolerated value, partially ensue as a result of the interaction of topological differences and a number of tolerance-related deviations.
These production-related differences between the individual burners can be corrected by a non-recurring homogenization. For this, the flame temperatures of the individual burners are measured and balanced by a passive throttling of the fuel supplies (see, for example, WO-A1-2005/010437). As measuring methods for flame temperature determination, the following known methods are currently available:
(1) Calculation of the adiabatic flame temperature on the basis of spectroscopic measurements (see, for example, U.S. Pat. No. 6,318,891).
(2) By indirect measuring via                (a) the wall temperature of the burner        (b) the NOx emission of the burner        (c) the CO2 content or O2 content of the fuel gas (lambda probe).        
(3) Measuring the temperature via the chemiluminescence intensity of the flame, for example the chemiluminescence of the NOx molecules (see, for example, U.S. Pat. No. 5,670,784).
The optimization process often fails in practice because of the large number of burners which are to be optimized and currently also accommodated in a plurality of combustion chambers, of which burners the flame temperature can only be very slowly determined at the same time. The aforementioned methods for determining the flame temperature, except for the chemiluminescence intensity methods, require a typical measuring duration from about ten seconds to one minute. This time must be compared with the effort for a homogenization of a multi-burner system. A homogenization of N burners, during a mutual influencing of the burners, corresponds to an optimization of a system with N parameters. The measuring effort for such an optimization, even with efficient methods, shifts in the order of magnitude of N2. This leads to more than one day being required for a complete balancing of a system with 50 burners.
Temperature determination on the basis of the intensity of the chemiluminescence was proposed at a very early stage. The intensity of the chemiluminescence I, which is collected by the lens, depends upon the flame temperature T, via a modified Arrhenius law:
                              I          ⁡                      (            T            )                          =                  A          ·                      Φ            0                                          (                                  T                  -                                      T                    0                                                  )                            τ                                                          (        1        )            
In this, Φ0 refers to the radiation density for a flame at the temperature T0. This intensity, as the characteristic value τ, depends upon the composition of the fuel and upon the pressure. The measured intensity I, however, is also determined by the transmissivity and the aperture of the lens, which are summed in the surface parameter A. If all parameters are known, then the temperature determination can be carried out very quickly on the basis of the intensity of the chemiluminescence. Even in the case of burners under pressure (30 bar) and at temperatures of 1200° C., the chemiluminescence intensity is sufficient to be measured with a frequency of up to 10 kHz.
In practice, the temperature determination via the intensity of the chemiluminescence is impractical since the chemiluminescence is very sensitively dependent upon the composition of the air (moisture) of the fuel, and also upon the pressure in the combustion chamber. Even the restriction to individual wave length ranges such as OH*, CH* or NO* brings no improvement at all in this case since the dependency upon the fuel composition occurs in the case of any radical. Moreover, an intensity determination always suffers from a transmission loss of the lens as a result of moisture which can enter quickly at some time during combustion processes.